Tobacco leaf stems have generally been found to be objectionable in smoking tobacco blends, particularly in blends for cigars and cigarettes. Stems have undesirable burning qualities and their stiffness may lead to deformed or punctured wrappers. In making smoking tobacco products, it is therefore customary to subject tobacco leaf to a threshing operation to separate the stem from the remainder of the leaf.
The stems, after separation, may be processed to produce products useful in smoking products. For example, they may be ground, mixed with fines, and converted into synthetic leaf, or the whole stem may be converted to useable filler material by an enzymatic process. Relatively long pieces of stem are more suitable for processing into a smoking product. In addition, relatively long pieces of stem are easier to remove from the rest of the leaves. Thus, any commercially suitable threshing process must result in the production of relatively long stems.
The remainder of the leaf, the lamina, is the portion that is the most important in production of smoking tobacco products. High grade tobacco products contain little stem and the lamina is by far the most valuable part of the leaf. Consequently, it is desirable to remove the stems with as little attached lamina as possible.
It is also commercially desirable to keep the lamina in relatively large pieces. Large pieces may be handled and shredded more easily during processing into high quality tobacco filler for cigars and cigarettes. Even more importantly, the destemming process must keep the production of fines, the dustlike particles of lamina, to a minimum. Tobacco fines, unless processed into reconstituted tobacco sheets, are not suitable for use in tobacco products. Thus, production of large amounts of fines represents a significant loss of valuable lamina.
In known tobacco leaf destemming processes, the leaf stems are separated from the leaf lamina by first subjecting the leaves to a mechanical threshing action of sufficient duration and intensity to completely detach the lamina from the stems. The resulting stem-lamina mixture is then subjected to a classification step. In typical threshers, lamina is separated from stems or veins by the action of one or more toothed rotors beating against stationary teeth, or by the action of counter-rotating toothed rotors, or by the action of a toothed rotor beating against a perforated cage or basket or by the action of a toothed rotor beating first against stationary teeth and then against a perforated cage or basket.
Because of the relatively ductile nature of the tobacco lamina, it will not easily break away from the stems. Therefore multiple impacts by the rotors are required to tear and rip the lamina and the stem must undergo violent flexing during this phase of the process, if all the lamina is to be removed from the stems and large veins.
The threshing processes currently in use, even if carefully controlled, result in the production of a preponderance of small pieces of lamina. In addition, an unacceptable amount of tobacco fines is produced, because of the pulverizing action of the toothed rotors and the multiple impacts required to completely detach all the lamina. The multiple impacts and violent flexing action also result in the production of broken and undesirably short stems.
In addition, current threshing processes often require, as an initial step, the addition of significant amounts of water, to permit handling of the leafs without causing undue fragmentation of the lamina. This water has to be removed in a drying step, subsequent to classification, before the tobacco lamina can be processed into a marketable product.
The present invention provides a technique whereby the stems can be easily detached from the tobacco leaves and then separated from the lamina. The technique of the present invention results in the production of relatively large pieces of lamina and long stems and veins. Furthermore, the process of the present invention does not require the addition of large amounts of water to prepare the tobacco for the stem separation process.